1. Field of the Invention
This invention relates to mufflers, and in particular to high efficiency, sound-cancelling flow-through mufflers arranged to reduce fuel consumption by decreasing back-pressure and therefore load on an engine.
In a first set of preferred embodiments, the mufflers feature a linear inner passage and a curved or zig-zag shaped outer passage that wraps around or surrounds the inner passage and is situated between the inner passage and an exterior wall of the muffler. The outer passage has a length that equals one-half the wavelength of sound to be canceled, or multiples of one-half wavelength, such that sound waves exiting the outer passage destructively interferes with or cancels sound waves directly exiting the inner passage to a sound-cancellation or conversion chamber at the outlet of the passages.
In a second set of preferred embodiments, the mufflers feature a linear inner passage and a curved or zig-zag shaped outer passage that wraps around or surrounds the inner passage and is situated between the inner passage and the exterior wall of the muffler, and that has a length equaling one-quarter the wavelength of sound to be cancelled, or multiples of the quarter wavelength, such that sound waves propagating in the outer passage and reflected from an end of the outer passage cancel sound waves propagating in the inner passage.
In each of the sets of preferred embodiments, the walls of the inner and outer passages are arranged to extract heat from exhaust gases in the passages, and to conduct the heat to an exterior surface of the muffler. The heat conducted to the exterior surface of the muffler may be advantageously be used for electricity generation, according to the principles described in copending U.S. patent application Ser. No. 10/211,701, filed Aug. 26, 2005, or for other purposes such as in a reformer that converts hydrocarbons such as diesel fuel into hydrogen gas.
In order to draw exhaust gases through the outer passage, the inner passage may have a Venturi nozzle that opens into the sound cancellation conversion chamber, thereby increasing the rate of heat extraction from the exhaust stream.
2. Description of Related Art
Noise silencing mufflers are well-known for reducing the volume of sound that accompanies the flow of exhaust gases from an internal combustion engine. The most commonly-used type of muffler incorporates a sound absorbing material through which exhaust gases are passed. However, the material through which the gases are passed inherently cause back-pressure to be exerted on the engine. Excess back-pressure increases fuel consumption and decreases engine performance.
To decrease back-pressure on the engine and increase efficiency, it is also known to provide mufflers which incorporate circuitous passages that isolate and attenuate sound waves generated by the exhaust stream, rather than relying on sound absorbing materials. In order to minimize turbulence or eddy currents that increase back pressure and noise, many of the circuitous passage designs divide the incoming exhaust stream into multiple paths, and converge the paths at the outlet.
Examples of flow-through mufflers with circuitous passages are disclosed in U.S. Pat. Nos. 3,786,896; 4,809,812; 5,444,197; 6,364,054; and 6,296,074. Of particular interest are U.S. Pat. No. 6,364,054, which discloses a muffler having expansion chambers and reduced openings that decrease sound volume by conversion of a part of the sound energy to heat energy, and which modify the frequency of sound passing through the muffler to provide tuning. Also of particular interest are U.S. Pat. No. 6,089,347, in which various dividers and partitions are arranged so that sound is attenuated in the spaces between the partitions as exhaust gases are directed past the outward ends of the partitions, the lengths of the spaces between the partitions being chosen to tune the muffler to affect selected frequencies, and U.S. Pat. No. 6,296,074, which discloses a sound cancelling muffler, in which the exhaust stream is separated into two different passages having different lengths so that at the point of conversion, the acoustic waves generated by the exhaust gases are out of phase and actually cancel, as opposed to merely being attenuated.
While these flow-through mufflers have the effect of reducing back-pressure on the engine, however, they are relatively expensive to manufacture and have not achieved widespread use, except in specialty applications. The reason is that they are relatively complicated to construct, and only cancel a limited range of frequencies. The sound cancelling muffler disclosed in U.S. Pat. No. 6,296,074 utilizes an especially complex structure, which not only is expensive to construct but also takes up a relatively large volume.
Like U.S. Pat. No. 6,296,074, the present invention seeks to provide sound cancellation by splitting and converging the exhaust stream in such a way that sound waves propagating in the diverging streams converge out of phase and therefore cancel. However, the invention differs from the arrangement disclosed in U.S. Pat. No. 6,296,074 in that one of the passages is a linear inner passage, and the second passage is an outer passage that wraps around or surrounds the inner passage and that is situated between the inner passage and an exterior wall of the muffler. This arrangement not only provides sound cancellation in a minimal volume with relatively simple structure, but also provides a medium to conduct heat to the exterior of the muffler for use in thermo-electric generation or other purposes.
According to one aspect of the invention, a Venturi nozzle is used to draw exhaust gases through the outer passage, thereby improving heat extraction. The use of Venturi nozzles to manipulate airflow through an enclosed space is of course known, but not in combination with a sound-cancelling muffler. Instead, Venturi nozzles are conventionally used to prevent propagation of acoustic waves past the throat of the nozzle by increasing the speed of the sound waves to sonic levels so that sound can no longer propagate, by reflecting sound waves off the walls of a taper or at the narrow opening, or by reducing pressure fluctuations downstream of the nozzle. Increasing the rate of exhaust gas flow through an outer sound-cancelling passage that wraps around the inner passage is not a consideration.
Examples of silencing devices that utilize Venturi nozzles, but not to draw gases through a heat-extraction passage, include the multiple stage Venturi muffler is disclosed in U.S. Pat. No. 1,611,475, the engine air intake Venturi silencers disclosed in U.S. Pat. Nos. 5,821,475 and 5,530,214, and the exhaust outlet Venturi silencer disclosed in U.S. Pat. No. 5,174,113. In addition, mufflers that use the Venturi effect to cause flow diversion and cancellation of acoustic pressure waves upon recombination are disclosed in U.S. Pat. Nos. 3,672,464 and 4,361,206, and an exhaust muffler that uses the accelerated exhaust gases to cause inflow of outside air is disclosed in U.S. Pat. No. 6,220,387. A final example of a muffler that claims to use a “Venturi” effect is the SpinTech muffler described in the websites [http//www] .off road.com/ford/flashback/spintech/spin.html and galaxieclub.com/spin/spintech.html, which is said to scavenge exhaust gases by creating a flow increase and trapping sound energy in spiral “sound traps” within the main exhaust passage.
In effect, the present invention represents a combination of the principle of heat conversion taught by the above-cited U.S. Pat. No. 6,364,054 and the sound attenuation or cancellation principles of U.S. Pat. No. 6,089,347, U.S. Pat. No. 6,296,074 and other prior patents, in a way that offers a substantially simplified construction and that enables increased versatility, including the ability to utilize the muffler for electricity generation and even emissions reduction, and to enable the muffler to be more easily arranged to compensate for the different frequencies generated at different engine speeds and loads. None of the prior systems, including those with sound cancellation and those that utilize Venturi nozzles suggests such a combination, or achieves the advantages in terms of simplicity of construction, efficiency, and sound reduction of the present invention.